#include <unistd.h>
#include <fcntl.h>
#include <sys/wait.h>
#include "syscall.h"
#include "pthread_impl.h"

struct ctx {
    int fd;
    const char *filename;
    int amode;
    int p;
};

static int checker(void *p)
{
    struct ctx *c = p;
    int ret;
    if (__syscall(SYS_setregid, __syscall(SYS_getegid), -1)
        || __syscall(SYS_setreuid, __syscall(SYS_geteuid), -1))
        __syscall(SYS_exit, 1);
    ret = __syscall(SYS_faccessat, c->fd, c->filename, c->amode, 0);
    __syscall(SYS_write, c->p, &ret, sizeof ret);
    return 0;
}

int faccessat(int fd, const char *filename, int amode, int flag)
{
    if (flag) {
        int ret = __syscall(SYS_faccessat2, fd, filename, amode, flag);
        if (ret != -ENOSYS) return __syscall_ret(ret);
    }

    if (flag & ~AT_EACCESS)
        return __syscall_ret(-EINVAL);

    if (!flag || (getuid()==geteuid() && getgid()==getegid()))
        return syscall(SYS_faccessat, fd, filename, amode);

    char stack[1024];
    sigset_t set;
    pid_t pid;
    int status;
    int ret, p[2];

    if (pipe2(p, O_CLOEXEC)) return __syscall_ret(-EBUSY);
    struct ctx c = { .fd = fd, .filename = filename, .amode = amode, .p = p[1] };

    __block_all_sigs(&set);
    
    pid = __clone(checker, stack+sizeof stack, 0, &c);
    __syscall(SYS_close, p[1]);

    if (pid<0 || __syscall(SYS_read, p[0], &ret, sizeof ret) != sizeof(ret))
        ret = -EBUSY;
    __syscall(SYS_close, p[0]);
    __syscall(SYS_wait4, pid, &status, __WCLONE, 0);

    __restore_sigs(&set);

    return __syscall_ret(ret);
}
